During the initial stage of well drilling through the earth's surface, regions of soil, sand, gravel, loose rock and other unconsolidated materials are encountered. In order to stabilize the casing string that surrounds the production tubing string in this region of unstable subsurface material, an outer casing is lowered with the progressing drill bit. When a more stable formation is reached, the outer casing terminates and an inner casing is then lowered to complete the drilling.
The outer casing may extend to a depth of 1,000 feet/330 m, or more, and is required to provide a barrier for the drilling operation and protect and stabilize the inner casing at the upper layer of the earth's surface where the subsurface is unconsolidated material. Once the drilling has reached a more compacted portion of the formation, the inner casing alone is lowered to the final drilling depth, which may be 4,000 feet/1300 m, or more. The inner casing is stabilized and rigidly secured in place by cementing the annular space between the two casings.
The purpose of a stage-cementing tool is to enable the operator to fill the annulus between the inner and outer surface casing strings with cement slurry when there is a lost circulation zone below the bottom of the outer casing. A lost circulation zone is one in which a cement slurry, drilling mud or other fluids cannot be contained in the well bore and are dissipated and lost in the surrounding formation. This is an undesirable condition and must be rectified.
One conventional stage-cementing tool consists of an inflatable packer element and a diverting tool (DV tool) above the packer. The tool is connected to the inner casing and run in the well to a depth of 50 to 100 ft above the bottom of the outer casing.
A heavy metal object, referred to in the art as a “metal bomb”, is dropped in the casing. The bomb falls freely in the drilling fluid in the casing and seats in the stage-cementing tool. Hydraulic pressure is applied from the surface to shift a sleeve and open a port in the stage cementing tool. Drilling fluid is pumped into the port to inflate the packer of the stage tool and form a seal with the outer casing. Higher pressure is then applied to open ports in the diverting tool above the packer. A known volume of cement slurry is pumped down the inner casing. A closing plug is dropped into the casing and drilling fluid is pumped to displace the plug and cement. The cement enters the casing annulus through the open ports in the DV tool above the packer. When the closing plug reaches the stage tool it shifts a sleeve to close the ports in the DV tool. At this time, the casing annulus is full of cement from the stage tool to the surface. The inflated packer forms a seal with the outer casing to prevent the cement slurry from falling into the lost circulation zone below the packer.
The following problems can develop when using a conventional stage-cementing tool:                1. The port to inflate the packer element fails to open. When this occurs, the packer cannot be inflated to form a seal with the outer casing and any cement pumped above the packer will fall down into the lost circulation zone below the stage tool. The casing annulus will remain full of drilling fluid or water.        2. The port in the diverting tool fails to open. When this happens, cement slurry cannot be pumped into the annulus.        3. The inflated packer fails to carry the weight of the cement column above it. The seal between the inflated packer and the outer casing is lost and all the cement slurry falls down into the lost circulation zone below the packer. Again the annulus will remain full of drilling fluid or water.        4. The closing plug fails to close the ports in the DV tool after all the cement has been pumped into the annulus. In this case, the operator has to wait about seven hours until the cement hardens before resuming operations. The waiting time could cost the operator from $7000 to $10,000 at contemporary prices.        
Cement baskets are sometimes used instead of stage-cementing tools to place cement in the casing annulus. Cement baskets cannot hold a large load of cement and, therefore, they are normally run to shallow depths of about 300 to 400 feet from the surface. Cement baskets do not form a seal with the outer casing and cement slurry can pass through the arms of the basket. For this reason the cementing job is performed by pumping cement slurry into the annulus in three to four stages to fill the annulus to the surface. After each stage the cement is allowed to harden for 3 to 4 hours before pumping the next stage. This procedure consumes excessive amount of rig time and is therefore costly.
It is therefore an object of the present invention to provide an improved stage-cementing apparatus and method that reliably seals the annular space at the desired depth.
Another object of the invention is to provide a stage-cement tool that can be installed relatively quickly and that is sufficiently robust to support a column of cement slurry that is 1000 feet, or more, in height.